http://blogs.msdn.com/mikhailarkhipov/archive/2006/12/25/repairing-visual-web-developer-intellisense-issues.aspx

Try this:

Cause: change of Microsoft Ajax namespace from atlas to asp. Visual studio uses reflection to inspect Web site references such as ASP.NET framework, assemblies in bin, etc and generates XML schemas that drive intellisense. Generated schemas are cached and change in the namespace when assembly name stays the same may confuse intellisense schema generator. This may happen when you uninstall Atlas Beta 1 is and install recent Release Candidate.

Solution: Close VS, delete everything in

C:\Documents and Settings\USER\Application Data\Microsoft\VWDExpress\8.0\ReflectedSchemas

C:\Documents and Settings\USER\Application Data\Microsoft\Visual Studio\8.0\ReflectedSchemas

then restart VS or VWD.

Let me know if this helps.